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span: add lifetimebound attribute

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See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0936r0.pdf for
reference.

This helps to guard against dangling references caused by construction from
temporaries such as:

    Span<const int> sp(std::vector<int>{1,2,3});
This commit is contained in:
Cory Fields 2020-06-25 17:25:31 -04:00
parent 62733fee87
commit 1d58cc7cb0
1 changed files with 14 additions and 4 deletions
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18
src/span.h
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@ -18,6 +18,16 @@
#define ASSERT_IF_DEBUG(x) #define ASSERT_IF_DEBUG(x)
#endif #endif
#if defined(__clang__)
#if __has_attribute(lifetimebound)
#define SPAN_ATTR_LIFETIMEBOUND [[clang::lifetimebound]]
#else
#define SPAN_ATTR_LIFETIMEBOUND
#endif
#else
#define SPAN_ATTR_LIFETIMEBOUND
#endif
/** A Span is an object that can refer to a contiguous sequence of objects. /** A Span is an object that can refer to a contiguous sequence of objects.
* *
* It implements a subset of C++20's std::span. * It implements a subset of C++20's std::span.
@ -87,14 +97,14 @@ public:
* Note that this restriction does not exist when converting arrays or other Spans (see above). * Note that this restriction does not exist when converting arrays or other Spans (see above).
*/ */
template <typename V> template <typename V>
constexpr Span(V& other, constexpr Span(V& other SPAN_ATTR_LIFETIMEBOUND,
typename std::enable_if<!is_Span<V>::value && typename std::enable_if<!is_Span<V>::value &&
std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value && std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value &&
std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr) std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
: m_data(other.data()), m_size(other.size()){} : m_data(other.data()), m_size(other.size()){}
template <typename V> template <typename V>
constexpr Span(const V& other, constexpr Span(const V& other SPAN_ATTR_LIFETIMEBOUND,
typename std::enable_if<!is_Span<V>::value && typename std::enable_if<!is_Span<V>::value &&
std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value && std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value &&
std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr) std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
@ -154,9 +164,9 @@ public:
/** MakeSpan for arrays: */ /** MakeSpan for arrays: */
template <typename A, int N> Span<A> constexpr MakeSpan(A (&a)[N]) { return Span<A>(a, N); } template <typename A, int N> Span<A> constexpr MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
/** MakeSpan for temporaries / rvalue references, only supporting const output. */ /** MakeSpan for temporaries / rvalue references, only supporting const output. */
template <typename V> constexpr auto MakeSpan(V&& v) -> typename std::enable_if<!std::is_lvalue_reference<V>::value, Span<const typename std::remove_pointer<decltype(v.data())>::type>>::type { return std::forward<V>(v); } template <typename V> constexpr auto MakeSpan(V&& v SPAN_ATTR_LIFETIMEBOUND) -> typename std::enable_if<!std::is_lvalue_reference<V>::value, Span<const typename std::remove_pointer<decltype(v.data())>::type>>::type { return std::forward<V>(v); }
/** MakeSpan for (lvalue) references, supporting mutable output. */ /** MakeSpan for (lvalue) references, supporting mutable output. */
template <typename V> constexpr auto MakeSpan(V& v) -> Span<typename std::remove_pointer<decltype(v.data())>::type> { return v; } template <typename V> constexpr auto MakeSpan(V& v SPAN_ATTR_LIFETIMEBOUND) -> Span<typename std::remove_pointer<decltype(v.data())>::type> { return v; }
/** Pop the last element off a span, and return a reference to that element. */ /** Pop the last element off a span, and return a reference to that element. */
template <typename T> template <typename T>